Validation of simplified quantitative methods for 3*deoxy-3*-[18F]fluorothymidine Positron Emission Tomography ([18F]FLT PET) in patients with non-small cell lung cancer treated with tyrosine kinase inhibitor (gefitinib of erlotinib).

[18F]FLT PET is widely investigated as a proliferation marker in oncology
[Bading et.al. J Nucl Med. 2008 Jun;49 Suppl 2:64S-80S]. [18F]FLT follows the
salvage pathway of endogenous thymidine in the cell. However, unlike thymidine,
[18F]FLT is trapped in the cytosol and is not incorporated into DNA. Published
data on [18F]FLT PET are heterogeneous and it is not clear to what extent this
relates to different pharmacokinetic characteristics, biological changes, image
resolution, or quantification methods. Pharmacokinetic (pk) modelling of PET
tracers is the gold standard but it requires arterial blood sampling and
dynamic scanning. In case of [18F]FLT, the net uptake rate constant of
[18F]FLT, Ki, determined by non-linear regression (NLR) of an irreversible
two-tissue compartment model is typically used as the gold standard.
Alternatively, kinetic filtering methods have been proposed, requiring
procedures of similar complexity [Gray et.al. Phys Med Biol. 2010 Feb
7;55(3):695-709]. This procedure is not suited for daily clinical practice and
it is not suited for whole body acquisitions.
Two simplified methods often are used to (semi-)quantitatively assess [18F]FLT
uptake: graphical (Patlak) analysis [Patlak et.al. J Cereb Blood Flow Metab
1985 5:584-590] and standardized uptake values (SUV). Patlak analysis assumes
irreversible trapping in tissue, and its accuracy thus depends on the
assumption that no significant dephosphorylation occurs within the time course
of the study. Both NLR and Patlak measure net uptake of [18F]FLT, taking into
account the concentration of tracer in plasma during the course of the study.
Only NLR, however, allows for measurements of individual rate constants between
compartments and for an implicit correction for blood volume in the tissue of
interest. SUV is the ratio of tissue concentration and injected activity at a
certain time after administration of the tracer. It does not take tracer
kinetics into account, but has the advantage that it is a single scan procedure
that does not require plasma data. In daily clinical practice, a static PET
scan in whole body mode is the preferred clinical procedure. In this context
only SUV is feasible.
For simplified uptake measures to be valid for monitoring response or
predicting outcome, their relationship with the more accurate outcome measures
of full kinetic analysis must be similar before and after therapy [Hoekstra
et.al. Eur J Nucl Med. 2000 Jun;27(6):731-43]. However, systemic therapy might
alter the correlations between NLR, Patlak and SUV, as has previously been
shown for [18F]fluorodeoxyglucose [Cheebsumon et.a. Eur J Nucl Med Mol Imaging.
2011 May;38(5):832-42]. This could be due to changes in tumour blood flow,
blood volume, or plasma clearance of the tracer. The changes are accounted for
in full kinetic analysis (NLR), but not in the use of SUV. In untreated cancer
patients it has been suggested that SUV is a reasonable alternative for pk
modelling [Kenny et.al. Cancer Res 2005 65:10104-10112].

Interpretation of the [18F]FLT signal may be a function of timing of PET during
therapy: early during therapy [18F]FLT measurements seem to correlate better
with proliferation than late ones [Salskov et.al. Semin Nucl Med. 2007
Nov;37(6):429-39][Benz et.a. Cancer. 2011 Oct 21]. Dynamic scanning at two
time-points in the same patient may help to elucidate whether these apparently
conflicting findings relate to the applied PET methodology.

Gefitinib and erlotinib are both orally administered tyrosine kinase inhibitor
of the epidermal growth factor receptor (EGFR). Tyrosine kinase inhibitors are
of special benefit in patients with NSCLC with an activating EGFR tyrosine
kinase mutation. Response rates in a first line setting range from 4-32% in
unselected patients and from 55-90% in patients with activating EGFR mutations
[Costanzo et.al. J Biomed Biotechnol. 2011;2011:815269].

Primary objective: association between pharmacokinetic modeling of [18F]FLT PET
and simplified measures during treatment with gefitinib in patients with NSCLC
to validate the use of simplified measures during treatment with gefitinib of
erlotinib

Secondary objectives:
1. comparison of baseline kinetic and simplified [18F]FLT measures
2. validation of kinetic filtering methodology
3. impact of perfusion ([15O]H2O) on [18F]FLT uptake prior to therapy and
during gefitinib or erlotinib

Prospective observational multicentre study including 10 eligible patients with
NSCLC (tumour size at least 3 cm) will be scanned with [18F]FLT PET on three
separate occasions; first within 7 days prior to treatment, subsequently 7 and
28 days after the first dose of gefitinib or erlotinib. Patients will also be
scanned with [15O]H2O, prior to the [18F]FLT study, to investigate the impact
of perfusion during gefitinib. The dynamic PET scans within the thoracic region
will be performed on a PET-CT scanner. During PET, venous samples will be taken
at different time points. Dedicated in-house developed software will be used to
quantify kinetics. Personal and tumour characteristics will be registered (age,
sex, body weight and height, comedication).
Patients will be recruited by the VUmc, UMC St Radboud Nijmegen and local
hospitals, Amsterdam, The Netherlands.

The venous cannulas will be placed by highly qualified medical doctors of the
Department of Nuclear Medicine & PET Research. In spite of this, occasionally
these cannulas may cause a hematoma. During the two PET scans a maximum of 180
ml blood is taken.
A PET scan is a regular diagnostic imaging technique. Each study will be
conducted in compliance with the radiation safety guidelines of the department.
Based on results we obtained from biodistribution studies in rats, whole body
radiation after intravenous injection of 300 MBq [18F]FLT is approximately 6.5
mSv, including the low dose CT used for attenuation correction. The whole body
radiation of the 300 MBq [15O]H2O will be around 0.3 mSv. Patients will undergo
three [18F]FLT PET scans, together with [15O]H2O PET scanning. The total amount
of radiation burden will be approximately 20.4 mSv for the entire study. To
compare, every person living in the Netherlands receives a natural background
radiation dose of 2-2,5 mSv per year.
We are aware that the radiation burden for this study is high, but are of the
opinion that this is acceptable for this particular study (with this specific
population and high scientific impact). The study population has a median
survival of approximately 24-27 months [Morita et.al. Clin Cancer Res. 2009 Jul
1;15(13):4493-8], were the possible hazard of cancer induction by radiation is
after several years. In addition, the patients will very likely receive
chemoradiation therapy as part of the best standard of care. The radiation
burden from the treatment will be a multitude higher than the radiation dose
from the diagnostic work-up and the presently suggested study. In addition, the
results of this study will have great clinical benefit in using [18F]FLT PET-CT
as drug monitoring tool in the future, improving personalized therapy
strategies for cancer patients. We therefore consider the additional radiation
burden acceptable and we feel that it outweighs the scientific merit of results
that come from the suggested study.